The invention relates to the injection molding process, blow molding process or the like and in particular to a positive control non-return valve that controls the flow of molten plastic fluids. More specifically, the invention relates to an improved non-return valve assembly that allows the proper flow through said valve in one direction and positively stops the reverse flow of material therethrough.
The manufacturing process of injection molding is a primary form of manufacturing of plastic products in the world today. The demands of high molded part tolerances, dimensional stability, and shot-to-shot repeatability are increasing and better controls and mechanical components of the injection machines are required to meet the demands. A major component that contributes to this process of improvement is the plasticating unit and specifically, the non-return valve which is one of the components of the assembly. The non-return valve controls the volume of molten plastic material that is injected into the mold. Any imperfection in the operation of this component is reflected in the molded part. Imperfect molded parts cost the industry billions of dollars per year. An improvement in the non-return valve is needed to eliminate imperfect molded parts.
Injection Molding machines of the type in which the invention pertains to generally comprises an elongated helical plasticating screw which rotates and recripicates axially in a cylindrical bore of an elongated heated barrel for moving heated plastic material along the helical flight channels of the screw, from an inlet port to a discharge port where it is injected from a distribution chamber, through a nozzle and into a mold. The flow of the molten plastic material into the distribution chamber for subsequent discharge through the nozzle is controlled by the non-return valve.
A review of the prior art illustrates two primary methods/designs for controlling this process. The first is a sliding ring non-return valve which is illustrated in FIGS. 2A and 2B. The second is the ball check non-return valve which is illustrated in FIG. 2C. There of course are various alterations that change the performance slightly such as U.S. Pat. No. 5,151,282 to Dray, and my own U.S. Pat. No. 5,164,207, but none embody the changes necessary to correct the problems that plague the device as well as this invention does.
With the sliding ring design, FIG. 2A, U.S. Pat. No. 4,105,147 (1977), U.S. Pat. No. 4,643,665 (1985), U.S. Pat. No. 5,112,213 (1991), U.S. Pat. No. 4,850,851 (1989), U.S. Pat. No. 4,681,528 (1987) as examples, the sliding ring 88, becomes the shutoff mechanism which when the helical screw 10, moves foreword to displace plastic into the mold 42, engages with and forms a seal with the rear seat 92. The ring, hereby referred to as the shutoff mechanism, is free to float and is not connected to the helical screw or fluted retainer 90. This shutoff mechanism forms a fit within close proximity to barrel 12, to form a gap 48. Having a free floating unattached shutoff mechanism that is not connected to and does not rotate with either the helical screw or retainer and that is the component that forms the gap, is desirable for precise valve operation.
With the ball check valve design FIG. 2B, U.S. Pat. No. 5,097,864 (1992), the ball 98, becomes the shutoff mechanism upon screw forward movement, forming a seal with seat 102, thereby not allowing molten plastic back flow. Although this ball shutoff mechanism is free floating, it does not comprise the gap 48. The body 94, which is in close proximity to the barrel wall, rotates with the screw and acts as a bearing surface, creating frictional heat and causing adhesive wear problems between the outside diameter of the valve and the inside diameter of the barrel. This frictional heat and potential wear problem is undesirable. Even the Dray design and my own patent xe2x80x9cThe Auto-shutxe2x80x9d valve rotate with the screw.
Common problems associated with non-return valves of this type are; a) the passage ways allowed for material flow is restricted, not allowing for ample area for which to pass which reduces the throughput, slowing down the injection molding cycle. Also, a mechanical shearing action is exerted on the material which creates frictional heat for the material, sometimes exceeding it""s capacity. b) The sealing surfaces in either the ring type, ball type or Auto-shut design can become exposed to unmelted or partially melted polymeric material, inhibiting a good seal, which allows for material migration back through the valve. This lost volume of material will cause an imperfect product to be formed in the mold. Imperfect products require significant inspection costs for the manufacturer. c) The retaining areas that are designed to govern the forward movement of the shutoff mechanism during screw rotate, are forced against each other with a combination of hydraulic pressure and high rotational speeds causing them to wear adhesively. As wear occurs, the functionality of the valve begins to diminish, creating more imperfect products. d) The inability of the non-return valve and specifically, the shutoff mechanism to shutoff completely over a range of viscosity""s that is normally seen in a day-to-day operation of an injection molding plant. It is not uncommon for a injection molder to process a multitude of polymers each day on the same machine requiring a valve that can operate efficiently over this wide range of viscosity""s. It is our opinion that the major cause for valve shutoff failure is the differential of surface area of the shutoff mechanism from the proximal end to the distal end. The greater the differential in the positive direction, the greater the closing force, which is directly related to molded part quality. A lack of significant differential exists on most valves today, making them in-effective to operate with a range of viscosity""s. When this occurs, a change of components is necessary or a change in the entire assembly is necessary which creates downtime and labor costs.
Accordingly, several objects and advantages of the invention are listed herein including but not excluding:
a) to provide a non-return valve that uses mathematical and Theological analysis to calculate not only cross sectional area but polymeric shear rates when designing the stroke of the body and volumetric passageways through the valve. During the screw recovery phase of the injection molding cycle the molten plastic fluid is pumped along the flight channels until it engages the proximal end of the body, urging it forward and providing an annular passageway for the fluid to flow. The annular passageway leads to a cylindrical geometry that leads to a conical portion that leads to a plurality of cylindrical ports that leads through the valve exiting to the discharge chamber. Generous radii are provided along with smooth passageways so as not to induce shear concentration points. The design lends itself to ease of manufacturing;
b) to provide a non-return valve that through it""s design offers a significant mechanical advantage during the most critical shutoff phase of the injection molding cycle. By virtue of the inventions significant frontal area specifically the differential between the frontal area and the rearward area of the shutoff mechanism, the large mass and increase in the length of engagement, which all contribute to the valves closing force, the performance is significantly improved over the industry standard components. This significant increase in the closing force equates to energy that is used to finish melting the partially melted polymer and assure more consistent shutoff performance,
c) to provide a non-return valve that has a improved design of the retaining members that govern the forward movement of the shutoff mechanism. The invention""s design incorporates a large spherical seat area constructed of high wear resistant materials. The spherical geometry assures that there will be uniform and equal contact between the two mating components even though the axis of the body does not remain the same as the axis of the spherical retainer when the molten plastic fluid is being pumped through the valve. Perpendicular surfaces or conical surfaces which are commonly used do not assure even surface contact and hence lead to high stress concentration areas and premature wearing of the components. In addition, the significant frontal area of the shutoff mechanism that is forward of the discharge plane acts as a counter force which lessens the total force being applied between the two mating sealing areas, thereby extending the wear life. This fact is believed to be unique amongst the industry standard designs;
d) to provide a non-return valve whose member that is attached to, and rotates with, the plasticating screw, has a geometry that offers little torsional resistance to the molten plastic fluid and occasional solid phase polymeric material so as not to have the tendency to break or fail torsionally. The inventions rotating attached member, 70,80, is cylindrical in nature with a generally inscribed polygon installed for ease of removal. The polygon that is close in proximity to the diameter of the shaft is significantly less likely to resist the flow of the polymeric material than the industry standard non-return valves, thus transmitting torque to the shaft that ultimately leads to torsional failure;
e) to provide a non-return valve that has a significant mechanical advantage during the shutoff or closing phase of the injection molding cycle that allows it to operate over a wider range of viscosity""s, delivering shutoff performance that is unmatched with the known existing technology. By virtue of the inventions significant frontal area, specifically the differential between the frontal area and the rearward area of the shutoff mechanism, the large mass and increase in the length of engagement, which all contribute to the valves closing force, the performance is significantly improved over the industry standard components. The magnitude of difference in closing force is enough to supply shutoff performance over the wide range of viscosity""s that are encountered in the injection molding process, thereby eliminating the need to modify or replace the non-return valve.